Transcript
8.2.3 ELECTROMAGNETIC SPECTRUM Recent technological developments have allowed greater use of the electromagnetic spectrum
3.1 Describe electromagnetic waves in terms of their speed in space and their lack of requirement of a medium for propagation
Is a self-propagating wave – changing electric and magnetic fields which oscillate perpendicular to each other Travel at the speed of light – in a vacuum o Since velocity is the same for all EM waves, frequency and wavelength change ( )
Ray
Radio
Microwave
Infrared
Visible
UV
X-rays
to
to
to
to
to
Microwave ovens
Sun Warm objects (fire/people)
Sun Hot objects Lamps/Lasers
Sun Hot objects Sparks
X-ray tubes
λ (m) Sources
TV/radio
Gamma rays
Radioactive nuclei atoms Cosmic rays
3.2 Identify the electromagnetic wavebands filtered out by the atmosphere, especially UV, X-rays and gamma rays Ray
Effect of Atmosphere
Radio
Not absorbed, >102 m λ reflected by ionosphere
Microwave
Not absorbed
Infrared
Partially absorbed by water vapour and carbon dioxide
Visible
Not absorbed
Atmosphere has two main filters – ionosphere and stratosphere, humans live in troposphere Ionosphere, 50 to 500 km above Earth, composed of ionised gases – Regions D, E and F o D: 50-80 km, gamma rays and shorter λ Hard X-rays absorbed o E: 80-145 km, longer λ Soft X-rays absorbed o F: 145-300 km, short λ UV absorbed Stratosphere: longer λ UV absorbed
3.3 Identify methods for the detection of various wavebands in the electromagnetic spectrum Ray
Method of Detection
Radio
Radio receivers connected to aerials
Microwave
Aerials, satellites
Infrared
Skin, night vision goggles
Visible
Eye, photographic film, photo cells
UV
Photographic film, photocells, fluorescent chemicals
X-rays
Photographic film, fluorescent screen
Gamma rays
Photographic film, Geiger counter
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3.4 Explain that the relationship between the intensity of electromagnetic radiation and distance from a source is an example of the inverse square law:
Intensity (measured in lux, lx) is reduced by the inverse square of the distance A light source with 16,000 lx: o
2 metres away would be
o
3 metres away would be
3.5 Outline how the modulation of amplitude or frequency of visible light, microwaves and/or radio waves can be used to transmit information
RADIO WAVES
Energy carried by waves can be varied to transmit information by varying frequency or amplitude o Frequency Modulation (FM) or Amplitude Modulation (AM) o Information is carried through signals by superposition of a carrier wave – tuning frequency o Signal occupies a range of frequencies around the carrier frequency – bandwidth Receiver subtracts carrier wave from signal and interprets variation in frequency/amplitude – demodulation AM advantage – uses much narrower range of frequencies – more stations fit into limited bandwidth FM advantage – not dependent on amplitude changes, so strength of signal does not change (frequency difficult to change due to interference)
MICROWAVES
Greater available bandwidth (20,000 phone calls), higher transmitted energy (less spread out) Reception in buildings more difficult due to short λ, range affected by atmospheric conditions (oxygen)
LIGHT
High energy laser light using amplitude modulation (frequency bandwidth too small for light) Fibre optic cables required – only reliable to 200 m in open air, due to more interference (narrow frequency)
3.6 Discuss problems produced by the limited range of electromagnetic spectrum available for communication purposes
Congestion of frequencies – bandwidth allocations required Example: FM radio stations allocated with 0.2 MHz bandwidth – possible frequencies of 96.7, 96.9, 97.1, etc
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